Measuring instruments are often mounted in a contacting manner for testing chemical, mechanical, and optical properties of surfaces, for example, of lacquers. These include, for example, color and brightness measuring instruments, layer thickness measuring instruments, and hardness measuring instruments.
If a coating is applied to a substrate which is not fully set, the measuring head may contact the coating when mounting the measuring instrument. This causes the measuring head to become contaminated; further measurements may be distorted and the contamination may be entrained onto subsequent substrates. The measuring instrument may even become irreparably damaged. If the measuring head has contacted a not fully set coating, the measuring head must be cleaned. Reasons for not fully set coatings may include, for example, unoptimized formulations, incorrect hardening conditions, or interference in the hardening systems.
Furthermore, it is also possible that surfaces become glued together or contaminated during handling and storage of the objects. Contaminations may also occur due to handling systems or storage systems such as shelves or racks and the like.
Common damage patterns of insufficiently set coatings include, for example, liquid coating material, soft or smearing surfaces, or a solid, hardened film over soft or liquid coating material. The coating material, the surface, or the solid, hardened film may or may not be adhesive.
To test the drying state of a lacquer, generally simple, manual methods are used in the lab. For example, glass beads are spread onto the surface or, pursuant to DIN 53150, paper having a defined weight is pressed onto the surface. It is tested whether it is possible to remove the beads or the paper without damaging the coating or using excessive force. The damage to the coating is verified by a visual check.
In another method generally used in the lab, the surface of the coating is scratched using a fingernail, and the possibly occurring damage pattern is evaluated, for example, using scrapers or via color transfer onto the finger or by detaching a film from the coating. A solid film over liquid or soft, not fully set material may thus also be detected. This is important in particular in the case of lacquers which are set in the presence of UV radiation.
The disadvantage of the above tests is, however, that they are not easily automated; in addition, they are inaccurate and depend on the tester. In many cases, only a rough estimation of “OK” or “not OK” is carried out.